Refrigeration systems typically contain oil in the range of 0 to 8 percent mixed in the refrigerant for lubrication purposes. It is useful to be able to analyze the working fluid to determine the amount of oil present. It is also useful to be able to observe the fluid to determine the presence of other matter such as metal particles which are indicative of compressor wear. On the other hand, such measurements and observations should not result in loss of refrigerant from the system and should not interfere with the system operation.